From the perspective of current and future technology trends, 400 GE/1 TE technologies may be based on a baud rate of 25 G (25.78125 G). However, using an existing NRZ modulation format as a basis can increase only a quantity of parallel channels (400 GE/16 channels, and 1 TE/40 channels), and an excessively large quantity of channels may cause a reliability problem. Therefore, 400 GE/1 TE may be implemented under a condition that the quantity of channels is moderately increased and that a bit rate of each channel is improved, between which a compromise needs to be reached. For improvement in a single-channel rate, high-order modulation may be used, and to reach a compromise between complexity and costs, relatively feasible spectral efficiency is 2 bit/Hz to 4 bit/Hz. Therefore, the high-order modulation becomes one of key technologies in short-range interconnection.
As shown in FIG. 1, a block diagram of a high-order modulation method is presented, where two NRZ signals are first combined into a 4-level PAM-4 signals by using a PAM-4 encoder, and then two PAM-4 signals pass through two filters to obtain two orthogonal IQ signals, which are superimposed and then transmitted to a directly modulated laser. A baseband signal is migrated to a higher frequency band when passing through the filters.
At a receive end, after optical-to-electrical conversion and front-end amplification, the two orthogonal signals pass through filters that match the foregoing filters, and are restored to two 4-level PAM-4 signals, then the 4-level PAM-4 signals pass through a PAM demodulator, and are finally demodulated into 4 NRZ signals.
From a spectrum chart on the upper left, it can be seen that an electrical signal at a transmit end of a modulated signal has undergone electrical modulation once, and a spectral pattern thereof depends on a spectral pattern of the filters. Mapping of a received signal onto a constellation diagram is presented on the upper right.
However, the foregoing solution can ensure performance in need of at least 4 times of oversampling, so that baseband bandwidth is wasted, bandwidth requirements are high, and resistance to dispersion is relatively low. In addition, to achieve a modulation effect, a baseband signal is modulated to a center frequency in a conventional modulation method, and if the center frequency is greater than a spectral width of the signal, it can be seen from a spectrum that there is usually a waste in bandwidth in a low frequency part. From the spectrum chart in FIG. 1, it can be seen that the low frequency is not covered by the spectrum of the signal after passing through an adder.
An orthogonal frequency-division multiplexing (Orthogonal Frequency-Division Multiplexing, OFDM) technology is also a modulation technology that relates to multiple sub-channels, with a main idea that a channel is divided into several orthogonal sub-channels, high-speed data signals are converted into parallel low-speed sub-data streams, the parallel low-speed sub-data streams are modulated onto the sub-channels in frequency domains, and then the parallel low-speed sub-data streams are transformed by using inverse Fourier transform into time-domain signals for transmission. Orthogonal signals may be separated at a receive end by using a relevant technology. However, there is a problem of carrier synchronization, and Fourier transform is needed to perform relevant digital signal processing, which has a problem of algorithm complexity and a relatively high requirement for a peak-to-average ratio of signals.